Chapter 54 Community Ecology

Overview: Communities in Motion A biological community is an assemblage of populations of various species living close enough for potential interaction For example, the “carrier crab” carries a sea urchin on its back for protection against predators © 2011 Pearson Education, Inc.

Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved Ecologists call relationships between species in a community interspecific interactions Examples are competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and facilitation Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0) © 2011 Pearson Education, Inc.

Competition Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply © 2011 Pearson Education, Inc.

Competitive Exclusion Strong competition can lead to competitive exclusion, local elimination of a competing species The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place © 2011 Pearson Education, Inc.

Ecological Niches and Natural Selection The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche An ecological niche can also be thought of as an organism’s ecological role Ecologically similar species can coexist in a community if there are one or more significant differences in their niches © 2011 Pearson Education, Inc.

Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community © 2011 Pearson Education, Inc.

A species’ fundamental niche is the niche potentially occupied by that species A species’ realized niche is the niche actually occupied by that species As a result of competition, a species’ fundamental niche may differ from its realized niche © 2011 Pearson Education, Inc.

Character Displacement Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species An example is variation in beak size between populations of two species of Galápagos finches © 2011 Pearson Education, Inc.

Percentages of individuals in each size class Figure 54.4 G. fuliginosa G. fortis Beak depth 60 Los Hermanos 40 G. fuliginosa, allopatric 20 60 Daphne 40 Percentages of individuals in each size class G. fortis, allopatric 20 Figure 54.4 Character displacement: indirect evidence of past competition. 60 Santa María, San Cristóbal Sympatric populations 40 20 8 10 12 14 16 Beak depth (mm)

Predation Predation (+/– interaction) refers to an interaction in which one species, the predator, kills and eats the other, the prey Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison © 2011 Pearson Education, Inc.

Prey display various defensive adaptations Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls Animals also have morphological and physiological defense adaptations Cryptic coloration, or camouflage, makes prey difficult to spot © 2011 Pearson Education, Inc.

Figure 54.5 Examples of defensive coloration in animals. (a) Cryptic coloration (b) Aposematic coloration Canyon tree frog Poison dart frog (c) Batesian mimicry: A harmless species mimics a harmful one. (d) Müllerian mimicry: Two unpalatable species mimic each other. Hawkmoth larva Cuckoo bee Yellow jacket Green parrot snake Figure 54.5 Examples of defensive coloration in animals.

Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration Predators are particularly cautious in dealing with prey that display such coloration © 2011 Pearson Education, Inc.

In some cases, a prey species may gain significant protection by mimicking the appearance of another species In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model © 2011 Pearson Education, Inc.

In Müllerian mimicry, two or more unpalatable species resemble each other © 2011 Pearson Education, Inc.

Herbivory Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores © 2011 Pearson Education, Inc.

Symbiosis Symbiosis is a relationship where two or more species live in direct and intimate contact with one another © 2011 Pearson Education, Inc.

Parasitism In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process Parasites that live within the body of their host are called endoparasites Parasites that live on the external surface of a host are ectoparasites © 2011 Pearson Education, Inc.

Many parasites have a complex life cycle involving a number of hosts Some parasites change the behavior of the host in a way that increases the parasites’ fitness © 2011 Pearson Education, Inc.

Mutualism Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species A mutualism can be Obligate, where one species cannot survive without the other Facultative, where both species can survive alone © 2011 Pearson Education, Inc.

(a) Acacia tree and ants (genus Pseudomyrmex) Figure 54.7 (a) Acacia tree and ants (genus Pseudomyrmex) Figure 54.7 Mutualism between acacia trees and ants. (b) Area cleared by ants at the base of an acacia tree

Commensalism In commensalism (+/0 interaction), one species benefits and the other is neither harmed nor helped Commensal interactions are hard to document in nature because any close association likely affects both species © 2011 Pearson Education, Inc.

Facilitation Facilitation (/ or 0/) is an interaction in which one species has positive effects on another species without direct and intimate contact For example, the black rush makes the soil more hospitable for other plant species © 2011 Pearson Education, Inc.

(b) (a) Salt marsh with Juncus (foreground) Number of plant species 8 Figure 54.9 8 6 Number of plant species 4 2 Figure 54.9 Facilitation by black rush (Juncus gerardi) in New England salt marshes. (a) Salt marsh with Juncus (foreground) With Juncus Without Juncus (b)

Concept 54.2: Diversity and trophic structure characterize biological communities In general, a few species in a community exert strong control on that community’s structure Two fundamental features of community structure are species diversity and feeding relationships © 2011 Pearson Education, Inc.

Species Diversity Species diversity of a community is the variety of organisms that make up the community It has two components: species richness and relative abundance Species richness is the number of different species in the community Relative abundance is the proportion each species represents of all individuals in the community © 2011 Pearson Education, Inc.

A B C D Community 1 Community 2 A: 25% B: 25% C: 25% D: 25% A: 80% Figure 54.10 A B C D Community 1 Community 2 Figure 54.10 Which forest is more diverse? A: 25% B: 25% C: 25% D: 25% A: 80% B: 5% C: 5% D: 10%

H = –(pA ln pA + pB ln pB + pC ln pC + …) Two communities can have the same species richness but a different relative abundance Diversity can be compared using a diversity index Shannon diversity index (H) H = –(pA ln pA + pB ln pB + pC ln pC + …) where A, B, C . . . are the species, p is the relative abundance of each species, and ln is the natural logarithm © 2011 Pearson Education, Inc.

Molecular tools can be used to help determine microbial diversity Determining the number and abundance of species in a community is difficult, especially for small organisms Molecular tools can be used to help determine microbial diversity © 2011 Pearson Education, Inc.

RESULTS 3.6 3.4 3.2 Shannon diversity (H) 3.0 2.8 2.6 2.4 2.2 3 4 5 6 Figure 54.11 RESULTS 3.6 3.4 3.2 Shannon diversity (H) 3.0 2.8 Figure 54.11 Research Method: Determining Microbial Diversity Using Molecular Tools 2.6 2.4 2.2 3 4 5 6 7 8 9 Soil pH

Diversity and Community Stability Ecologists manipulate diversity in experimental communities to study the potential benefits of diversity For example, plant diversity has been manipulated at Cedar Creek Natural History Area in Minnesota for two decades © 2011 Pearson Education, Inc.

Figure 54.12 Figure 54.12 Study plots at the Cedar Creek Natural History Area, site of long-term experiments on manipulating plant diversity.

Communities with higher diversity are More productive and more stable in their productivity Better able to withstand and recover from environmental stresses More resistant to invasive species, organisms that become established outside their native range © 2011 Pearson Education, Inc.

Trophic Structure Trophic structure is the feeding relationships between organisms in a community It is a key factor in community dynamics Food chains link trophic levels from producers to top carnivores © 2011 Pearson Education, Inc.

A terrestrial food chain A marine food chain Figure 54.13 Carnivore Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Figure 54.13 Examples of terrestrial and marine food chains. Herbivore Primary consumers Zooplankton Plant Primary producers Phytoplankton A terrestrial food chain A marine food chain

Food Webs A food web is a branching food chain with complex trophic interactions © 2011 Pearson Education, Inc.

Humans Smaller toothed whales Baleen whales Sperm whales Elephant Figure 54.14 Humans Smaller toothed whales Baleen whales Sperm whales Elephant seals Crab- eater seals Leopard seals Birds Fishes Squids Figure 54.14 An Antarctic marine food web. Carniv- orous plankton Euphau- sids (krill) Cope- pods Phyto- plankton

Species may play a role at more than one trophic level Food webs can be simplified by Grouping species with similar trophic relationships into broad functional groups Isolating a portion of a community that interacts very little with the rest of the community © 2011 Pearson Education, Inc.

Sea nettle Juvenile striped bass Fish larvae Fish eggs Zooplankton Figure 54.15 Sea nettle Juvenile striped bass Figure 54.15 Partial food web for the Chesapeake Bay estuary on the U.S. Atlantic coast. Fish larvae Fish eggs Zooplankton

Limits on Food Chain Length Each food chain in a food web is usually only a few links long Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis © 2011 Pearson Education, Inc.

Most data support the energetic hypothesis The energetic hypothesis suggests that length is limited by inefficient energy transfer For example, a producer level consisting of 100 kg of plant material can support about 10 kg of herbivore biomass (the total mass of all individuals in a population) The dynamic stability hypothesis proposes that long food chains are less stable than short ones Most data support the energetic hypothesis © 2011 Pearson Education, Inc.

Number of trophic links Figure 54.16 5 4 3 Number of trophic links 2 1 High (control): natural rate of litter fall Medium: 1/10 natural rate Low: 1/100 natural rate Figure 54.16 Test of the energetic hypothesis for the restriction of food chain length. Productivity

Species with a Large Impact Certain species have a very large impact on community structure Such species are highly abundant or play a pivotal role in community dynamics © 2011 Pearson Education, Inc.

Dominant Species Dominant species are those that are most abundant or have the highest biomass Dominant species exert powerful control over the occurrence and distribution of other species For example, sugar maples have a major impact on shading and soil nutrient availability in eastern North America; this affects the distribution of other plant species © 2011 Pearson Education, Inc.

One hypothesis suggests that dominant species are most competitive in exploiting resources Another hypothesis is that they are most successful at avoiding predators Invasive species, typically introduced to a new environment by humans, often lack predators or disease © 2011 Pearson Education, Inc.

Keystone Species and Ecosystem Engineers Keystone species exert strong control on a community by their ecological roles, or niches In contrast to dominant species, they are not necessarily abundant in a community Field studies of sea stars illustrate their role as a keystone species in intertidal communities © 2011 Pearson Education, Inc.

Number of species present Figure 54.17 EXPERIMENT RESULTS Figure 54.17 Inquiry: Is Pisaster ochraceus a keystone predator? 20 15 With Pisaster (control) Number of species present 10 Without Pisaster (experimental) 5 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year

Observation of sea otter populations and their predation shows how otters affect ocean communities © 2011 Pearson Education, Inc.

Otter number (% max. count) Grams per 0.25 m2 Number per 0.25 m2 Figure 54.18 100 80 60 Otter number (% max. count) 40 20 (a) Sea otter abundance 400 300 Grams per 0.25 m2 200 100 (b) Sea urchin biomass Figure 54.18 Sea otter as a keystone predator in the North Pacific. 10 8 Number per 0.25 m2 6 4 2 1972 1985 1989 1993 1997 Year (c) Total kelp density Food chain

Ecosystem engineers (or “foundation species”) cause physical changes in the environment that affect community structure For example, beaver dams can transform landscapes on a very large scale © 2011 Pearson Education, Inc.

Figure 54.19 Figure 54.19 Beavers as ecosystem engineers.

Bottom-Up and Top-Down Controls The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels In this case, the presence or absence of mineral nutrients determines community structure, including the abundance of primary producers © 2011 Pearson Education, Inc.

The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level above In this case, predators control herbivores, which in turn control primary producers © 2011 Pearson Education, Inc.

Biomanipulation can help restore polluted communities In a Finnish lake, blooms of cyanobacteria (primary producers) occurred when zooplankton (primary consumers) were eaten by large populations of roach fish (secondary consumers) The addition of pike perch (tertiary consumers) controlled roach populations, allowing zooplankton populations to increase and ending cyanobacterial blooms © 2011 Pearson Education, Inc.

Polluted State Restored State Fish Abundant Rare Zooplankton Rare Figure 54.UN02 Polluted State Restored State Fish Abundant Rare Zooplankton Rare Abundant Figure 54.UN02 In-text figure, p. 1206 Algae Abundant Rare

Concept 54.3: Disturbance influences species diversity and composition Decades ago, most ecologists favored the view that communities are in a state of equilibrium This view was supported by F. E. Clements, who suggested that species in a climax community function as a superorganism © 2011 Pearson Education, Inc.

Other ecologists, including A. G. Tansley and H. A Other ecologists, including A. G. Tansley and H. A. Gleason, challenged whether communities were at equilibrium Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances A disturbance is an event that changes a community, removes organisms from it, and alters resource availability © 2011 Pearson Education, Inc.

Characterizing Disturbance Fire is a significant disturbance in most terrestrial ecosystems A high level of disturbance is the result of a high intensity and high frequency of disturbance © 2011 Pearson Education, Inc.

High levels of disturbance exclude many slow-growing species The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance High levels of disturbance exclude many slow-growing species Low levels of disturbance allow dominant species to exclude less competitive species © 2011 Pearson Education, Inc.

In a New Zealand study, the richness of invertebrate taxa was highest in streams with an intermediate intensity of flooding © 2011 Pearson Education, Inc.

Index of disturbance intensity (log scale) Figure 54.20 35 30 25 Number of taxa 20 15 Figure 54.20 Testing the intermediate disturbance hypothesis. 10 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Index of disturbance intensity (log scale)

The Yellowstone forest is an example of a nonequilibrium community The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance The Yellowstone forest is an example of a nonequilibrium community © 2011 Pearson Education, Inc.

(a) Soon after fire (b) One year after fire Figure 54.21 Figure 54.21 Recovery following a large-scale disturbance. (a) Soon after fire (b) One year after fire

Ecological Succession Ecological succession is the sequence of community and ecosystem changes after a disturbance Primary succession occurs where no soil exists when succession begins Secondary succession begins in an area where soil remains after a disturbance © 2011 Pearson Education, Inc.

Early-arriving species and later-arriving species may be linked in one of three processes Early arrivals may facilitate the appearance of later species by making the environment favorable They may inhibit the establishment of later species They may tolerate later species but have no impact on their establishment © 2011 Pearson Education, Inc.

Retreating glaciers provide a valuable field-research opportunity for observing succession Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics 1. The exposed moraine is colonized by pioneering plants, including liverworts, mosses, fireweed, Dryas, willows, and cottonwood © 2011 Pearson Education, Inc.

1941 1907 2 Dryas stage 1 Pioneer stage, with fireweed dominant 1860 Figure 54.22-4 1941 1907 2 Dryas stage 1 Pioneer stage, with fireweed dominant 5 10 15 1860 Kilometers Glacier Bay Figure 54.22 Glacial retreat and primary succession at Glacier Bay, Alaska. Alaska 1760 4 Spruce stage 3 Alder stage

Succession is the result of changes induced by the vegetation itself On the glacial moraines, vegetation lowers the soil pH and increases soil nitrogen content © 2011 Pearson Education, Inc.

60 50 40 Soil nitrogen (g/m2) 30 20 10 Pioneer Dryas Alder Spruce Figure 54.23 60 50 40 Soil nitrogen (g/m2) 30 20 Figure 54.23 Changes in soil nitrogen content during succession at Glacier Bay. 10 Pioneer Dryas Alder Spruce Successional stage

Human Disturbance Humans have the greatest impact on biological communities worldwide Human disturbance to communities usually reduces species diversity © 2011 Pearson Education, Inc.

Figure 54.24 Figure 54.24 Disturbance of the ocean floor by trawling.

Concept 54.4: Biogeographic factors affect community diversity Latitude and area are two key factors that affect a community’s species diversity © 2011 Pearson Education, Inc.

Latitudinal Gradients Species richness is especially great in the tropics and generally declines along an equatorial-polar gradient Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate © 2011 Pearson Education, Inc.

Temperate and polar communities have started over repeatedly following glaciations The greater age of tropical environments may account for their greater species richness In the tropics, the growing season is longer, so biological time runs faster © 2011 Pearson Education, Inc.

Climate is likely the primary cause of the latitudinal gradient in biodiversity Two main climatic factors correlated with biodiversity are solar energy and water availability They can be considered together by measuring a community’s rate of evapotranspiration Evapotranspiration is evaporation of water from soil plus transpiration of water from plants © 2011 Pearson Education, Inc.

Vertebrate species richness Figure 54.25 180 160 140 120 100 Tree species richness 80 60 40 20 100 300 500 700 900 1,100 Actual evapotranspiration (mm/yr) (a) Trees 200 Figure 54.25 Energy, water, and species richness. 100 Vertebrate species richness (log scale) 50 10 500 1,000 1,500 2,000 Potential evapotranspiration (mm/yr) (b) Vertebrates

Area Effects The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species A species-area curve of North American breeding birds supports this idea © 2011 Pearson Education, Inc.

Number of species (log scale) Figure 54.26 1,000 100 Number of species (log scale) 10 Figure 54.26 Species-area curve for North American breeding birds. 1 0.1 1 10 100 103 104 105 106 107 108 109 1010 Area (hectares; log scale)

Island Equilibrium Model Species richness on islands depends on island size, distance from the mainland, immigration, and extinction The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point © 2011 Pearson Education, Inc.

Figure 54.27 The equilibrium model of island biogeography. Immigration Extinction Immigration Extinction Immigration (small island) (near island) Extinction (large island) (far island) Extinction Immigration Rate of immigration or extinction Rate of immigration or extinction (large island) (far island) Immigration Rate of immigration or extinction Extinction (near island) (small island) Equilibrium number Small island Large island Far island Near island Number of species on island Figure 54.27 The equilibrium model of island biogeography. Number of species on island Number of species on island (a) Immigration and extinction rates (b) Effect of island size (c) Effect of distance from mainland

Studies of species richness on the Galápagos Islands support the prediction that species richness increases with island size © 2011 Pearson Education, Inc.

Number of plant species Area of island (hectares) Figure 54.28 RESULTS 400 200 100 Number of plant species (log scale) 50 25 10 Figure 54.28 Inquiry: How does species richness relate to area? 5 10 100 103 104 105 106 Area of island (hectares) (log scale)

Concept 54.5: Pathogens alter community structure locally and globally Ecological communities are universally affected by pathogens, which include disease-causing microorganisms, viruses, viroids, and prions Pathogens can alter community structure quickly and extensively © 2011 Pearson Education, Inc.

Pathogens and Community Structure Pathogens can have dramatic effects on communities For example, coral reef communities are being decimated by white-band disease © 2011 Pearson Education, Inc.

Community ecology is needed to help study and combat pathogens Human activities are transporting pathogens around the world at unprecedented rates Community ecology is needed to help study and combat pathogens © 2011 Pearson Education, Inc.

Community Ecology and Zoonotic Diseases Zoonotic pathogens have been transferred from other animals to humans The transfer of pathogens can be direct or through an intermediate species called a vector Many of today’s emerging human diseases are zoonotic © 2011 Pearson Education, Inc.

Identifying the community of hosts and vectors for a pathogen can help prevent disease For example, recent studies identified two species of shrew as the primary hosts of the pathogen for Lyme disease © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Avian flu is a highly contagious virus of birds Ecologists are studying the potential spread of the virus from Asia to North America through migrating birds © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.